Fundamentals of Spectral Imaging

In the pushbroom spectral imager, each cross-track spatial pixel (1, …, N) is dispersed perpendicular to the platform motion vector so that individual wavelength bins can be separately resolved by the M pixels of the N × M focal plane array.

Spectral imaging is an umbrella term that encompasses all techniques whereby the electromagnetic radiation recorded from a given scene is resolved according to wavelength. Historically, the number of channels into which the light is divided has determined the nomenclature. Hence, systems with less than 100 spectral channels are termed multispectral, those with between 100 and 1000 channels are hyperspectral, and those with more than 1000 channels are ultraspectral. Most of the systems used at Aerospace fall into the hyperspectral range, but it is worth noting that modern parlance is tending away from these arbitrary divisions in favor of the more inclusive terminologies, “spectral imager” and “imaging spectrometer.”

The vast majority of imaging spectrometers operate in the visible-through-shortwave-infrared (VSWIR) spectral region, which is dominated by reflected and scattered solar radiation. In contrast, the thermal infrared region, comprising the midwave infrared (MWIR) and LWIR, not only offers technical challenges distinct from those of the VSWIR, but also accesses an entirely different range of phenomenologies because at these wavelengths the solar contribution is negligible and measurements depend on thermal emission originating from the scene itself.

There is considerable overlap between the possible applications for each type of imager, but the reliance on dissimilar phenomenologies (and the fact that VSWIR systems are only operable in daylight, while thermal infrared systems can operate throughout the diurnal cycle) means that the two classes of instrument are essentially complementary. Because these imagers can resolve the spectral content of the radiance from a scene, it becomes possible to carry out sophisticated analysis of the resulting data stream to infer physical and chemical properties of the observed surface and the intervening atmosphere. Airborne and space-based spectral imagers are of primary importance in surface compositional mapping, land use and ecology, and atmospheric trace gas distribution.

Aerospace’s emphasis on hyperspectral thermal infrared imagery recognizes that full exploitation of such sensors lags well behind VSWIR, for which many commercial imaging services are now offered.